Why are digital specs always initially delivered as 300x250px?

For digital specs our team will always send a 300x250 as the sample with watermarks... to ensure we are on the right creative track with messaging, colors, logos, placement, etc. before creating the 4 standard sizes. Once you receive the sample banner and are ready to run with it, you would then mark it sold in your account to receive the high-resolution, unwatermarked version in all 4 sizes.  This basically mirrors how audio specs are submitted and returned.... as a spec first then ready "for air" once marked sold.